In a mobile communications system such as, for example, the LTE system, Long Term Evolution, the base station, i.e. the eNodeB, of a cell in the system performs the processes known as link adaptation of user equipments, UEs, and scheduling of UEs for transmission between the users and the eNodeB.
The link adaptation comprises choosing the MCS, (Modulation and Coding Scheme), for the traffic between a UE and the eNodeB. One parameter which needs to be determined in order to perform the link adaptation as well as the scheduling is the SINR, (Signal to Interference and Noise Ratio), of the channel which is used between the eNodeB and the UE. Since the link adaptation and scheduling are, per definition, performed for future time intervals, usually one or more TTIs, (Transmission Time Intervals), in an LTE system, the SINR for a channel is determined as a prediction, albeit with the use of the channel's history.
In order to compensate for unknown effects in the prediction such as SINR determination errors, fast fading channels, time varying interference, inter-symbol interference, receiver gain etc, a so called “SINR back-off” can be used, i.e. a determined SINR is decreased with a “back-off parameter”. The back-off parameter can either be constant or it can be adapted to the results of previous traffic, i.e. using “channel history”. In the latter case, the back-off parameter is typically increased if many transmission block errors occur, and decreased if no or few transmission block errors occur.
If an adaptive SINR back-off solution is used, several transmissions will be needed in order for the solution to “converge”, and if channel conditions vary rapidly, convergence may be impossible to reach.